For years, there has been a demand for further miniaturization of an electronic circuit using a power semiconductor or the like which handles high power, such as an inverter circuit and a power supply circuit, in association with the miniaturization of devices using such circuits.
In association with the miniaturization of the devices, a substrate has been developed for exclusive use for an electronic circuit which handles the high power, such as the inverter circuit and the power supply circuit, to be employed in an electric power steering apparatus (EPS) for use in, for example, vehicles such as automobiles.
In order to achieve the development of the substrate intended for the exclusive use for such a power semiconductor, an important issue is how to radiate the heat due to a loss of, for example, the power semiconductor mounted with high density. As a conventional dedicated substrate of this type, such as a power substrate using the power semiconductor, a substrate having a metal supporting plate made of, for example, aluminum with its surface laminated with a conductive foil (a copper foil) through a thin insulator layer, is used. A wiring pattern is formed on the substrate while the conductive foil is etched. Meanwhile, as a control substrate, for example, a layered resin substrate with four or more layers, having a dedicated microcomputer mounted thereon, is used. A control unit has two substrates, that is, a control substrate and a power substrate, as described above, stacked in two layers vertically.
Therefore, in order to reduce the size and the thickness of the control unit as described above, there is an increasing demand for compounding such the two substrates stacked vertically in two layers into a single multilayer substrate. The high-density mounting of a power module section (mainly an inverter circuit) constituting such an integrated resin multilayer substrate (single substrate) is further implemented by, for example, the incorporation of a redundant circuit and the duplication of sensors. Thus, the heat generated from the power module section is released into a housing (case) storing the substrate via a thermal interface material (TIM) filled in minute gaps.
With regard to such a structure, there exists a demand for narrowing, as far as possible, the above-described gaps so as to improve the heat conductivity. On the contrary, when the contact between the substrate and the case due to a warpage of the substrate is caused due to excessively narrow gaps, there is possibly a problem that an unnecessary electrical contact between the substrate and the case may occur.
In other words, since the multilayer substrate made of resin is used instead of the metal supporting plate in the formation of the power substrate into the integrated substrate with the multilayer structure compounded as described above, the heat radiation is reduced in the conventional power substrate being formed on the metal supporting plate made of the aluminum or the like. Therefore, the heat conduction from the electronic parts mounted on the resin substrate as described above is conducted from the electronic parts through a wiring layer made of the copper foil to the resin substrate and its adhesion layer, and further is conducted from the back surface of the substrate via the TIM to a heatsink of the case. However, generally, since a wiring pattern is also formed on the back surface of the substrate, in a case that the warpage of the substrate occurs, there is possibly a problem that the substrate escapes from the TIM and comes into the contact with the case, and further an electrical contact may occur at an undesired position between the substrate and the case.
In order to solve such a problem, as schematically shown in, for example, FIG. 17, a method of securing a large clearance (a gap between the integrated substrate and the case) is adopted so as to prevent the electrical contact at the undesired position between the case 1000C and a wiring surface on the back surface side (the lower side in FIG. 17) of the integrated substrate 1000B including through holes TH and so on.
However, when the large clearance is secured as described-above, a thermal resistance increases with it. Thus, the width of the TIM needs to increase so as to fill the large clearance. In addition, there has been a problem of a high manufacturing cost since the TIM of a higher performance (for example, a TIM having a high heat conductivity of 2.6 [W/mK]) involves a higher cost.
Furthermore, for example, the technique described in Japanese Published Unexamined Patent Application No. 2004-31495 A (Patent Document 1) and that described in Japanese Published Unexamined Patent Application No. 2005-142228 A (Patent Document 2) have been disclosed so as to solve the above-described problem.
Patent Document 1 describes that: “As the thickness of a heat radiation member is reduced as a means for improving radiation performance, a printed circuit board and an electronic control unit (ECU) housing may come into contact with each other to cause, for example, a short circuit due to the effect of a warpage of the printed circuit board and difficulties in maintaining processing accuracy of the ECU housing. Thus, there arises a problem of a malfunction of an electronic device.” Under the recognition of the above-described technical problem, an object of Patent Document 1 is “to provide a heat radiation structure of an electronic device capable of improving radiation performance by minimizing the distance between a printed circuit board and a housing, without causing a malfunction of the electronic device”.
In order to solve the above-described problem, the technique described in Patent Document 1 adopts the configuration that a contact prevention means for preventing a contact between a housing and a circuit board is formed, between the housing and the circuit board, at a position different from the position of a wiring pattern formed on the circuit board. The contact prevention means is formed on the circuit board or the housing as a protrusion having a predetermined height.
The technique described in Patent Document 2 adopts the following power module structure. Plural inverter modules are attached, with the gaps, to the upper surface of a single module base section. Wirings connected to the inverter modules are extended from the inverter modules to the outside of the module base section. A locking tool disposition section for locking a radiator plate contacting the lower surface of the module base section is provided between the inverter modules.